An outbreak of severe diarrhoea has killed at least 135 people in rural central Haiti and left sick hundreds more, who overwhelmed a crowded hospital on Thursday seeking treatment. Health workers suspected the disease was cholera, but were awaiting tests.

Hundreds of patients lay on blankets in a parking lot outside St Nicholas hospital in the port of St Marc with IVs in their arms for rehydration. As rain began to fall in the afternoon, nurses rushed to carry them inside.

Doctors were testing for cholera, typhoid and other illnesses in the Caribbean nation's deadliest outbreak since the earthquake in January that killed as many as 300,000 people.

Catherine Huck, the deputy country director for the UN Office for the Coordination of Humanitarian Affairs (OCHA), said the Caribbean nation's health ministry had recorded 135 deaths and more than 1,000 people infected.

"What we know is that people have diarrhoea, and they are vomiting, and [they] can go quickly if they are not seen in time," Huck said. She said doctors were still awaiting laboratory results to pinpoint the disease.

The president of the Haitian Medical Association, Claude Surena, said the cause appeared to be cholera, but added that had not been confirmed by the government.

"The concern is that it could go from one place to another place, and it could affect more people or move from one region to another one," he said.

Cholera is a bacterial infection spread through contaminated water. It causes severe diarrhoea and vomiting that can lead to dehydration and death within hours. Treatment involves administering a salt and sugar-based rehydration serum.

The sick come from across the rural Artibonite region, which did not experience significant damage in the 12 January quake but has absorbed thousands of refugees from the devastated capital 45 miles south of St Marc.

Some patients said they drank water from a public canal, while others said they bought purified water. All complained of symptoms including fever, vomiting and severe diarrhoea.

"I ran to the bathroom four times last night vomiting," said 70-year-old Belismene Jean Baptiste.

Trucks loaded with medical supplies including rehydration salts were to be sent from Port-au-Prince to the hospital, said Jessica DuPlessis, a spokeswoman for OCHA. Doctors at the hospital said they also needed more personnel to handle the flood of patients.

Elyneth Tranckil was among dozens of relatives standing outside the hospital gate as new patients arrived near death. "Police have blocked the entry to the hospital, so I can't get in to see my wife," Tranckil said.

Aid groups were mobilising to ship medicine, water filtration units and other relief supplies to the Artibonite region.

"We have been afraid of this since the earthquake," said Robin Mahfood, the president of Food for the Poor, which was preparing to airlift donations of antibiotics, oral dehydration salts and other supplies.

The US embassy in Port-au-Prince issued an advisory note urging people to drink only bottled or boiled water and eat only food that has been thoroughly cooked.